1. Field of the Invention
The present invention relates to an image forming apparatus, such as an MFP (Multi-Function Peripherals) that is a digital multifunction machine, a copier, and a printer, and to a control method, and suppresses abnormal temperature rises while realizing a shortening of warm-up time and an improvement in printing speed.
2. Description of the Related Art
In image forming apparatus such as MFPs, copiers, and printers, usually a fixing device is used because a toner image is formed on paper and the toner image is fixed to the paper. The fixing device includes a heat roller for heating and a pressure roller, and the unfixed paper is conveyed between this pair of rollers. Incidentally, because the fixing device is cool when the power is switched ON and the like, the fixing device is warmed by heating the heat roller, but there has been the problem that it takes time for the fixing device to become warm, and this warm-up time is long. Further, in order to improve the printing speed, the conveyance of the paper may conducted at a high speed, but a conveyance motor that can be driven at a high speed becomes necessary.
For this reason, in order to shorten the warm-up time and increase the printing speed, a heater with a large amount of heat and a conveyance motor that can be driven at a high speed may be combined, but when the heater and the conveyance motor are driven at the same time, there is concern that the power consumption will exceed the rating. Moreover, a complicated and expensive control circuit has been used as a safety measure when the temperature of the heater exceeds a stipulated value.
In the image forming apparatus disclosed in Japanese Patent Application Publication No. 11-194648, an example is described where a voltage output circuit that supplies power to the heater of the fixing device is disposed, and the applied voltage is set high when the heater is initially lighted. Further, an example is described where an increase in the power consumption is suppressed by shutting down or reducing the burden such as the motor only at times when the applied voltage is high.
However, in this example, it is necessary to vary the applied voltage using a control IC in order to raise the applied voltage when the heater is initially lighted, and the circuit configuration has been complicated and expensive. Further, because there is one heater, the temperature of the heat roller in the fixing device does not become uniform, and precise temperature control has been impossible when the temperature of the heat roller is different between its central portion and both sides.
Further, in the fixing device disclosed in Japanese Patent Application Publication No. 7-325500, an example is described where the fixing device includes a heat roller and a pressure roller, and plural halogen lamps are disposed inside the heat roller. In this example, the surface temperature of the heat roller is made uniform by the plural halogen lamps inside the heat roller.
However, no consideration is given to controlling the conveyance motor. Consequently, a measure to reduce power consumption that includes both controlling the heater and driving the conveyance motor is not disclosed.